Polyurethane coating compositions generally include polyurethane polymers having chemically-stable, polar urethane linkages that are capable of hydrogen bonding. As such, cured films formed from polyurethane coating compositions may have good mechanical stability and exterior weatherability. Further, the cured films may exhibit increased flexibility when synthesized from a medium molecular weight polymeric diol such as a polyester polyol or a polyether polyol that has a number average molecular weight of from about 500 g/mol to about 4,000 g/mol.
Polyurethane polymers generally include a backbone or chain having two ends, and may be combined with a crosslinker to form the polyurethane coating composition. More specifically, the polyurethane polymer may have a hydroxyl functional group present at both ends of the backbone and the hydroxyl functional group may be reactive with the crosslinker. However, since a linear polyurethane polymer may only include two hydroxyl functional groups, i.e., one at each end of the backbone or chain, crosslinking density may be low.
Carbamate functional groups may also be introduced at the ends of the polyurethane polymer backbone by reacting terminal isocyanate groups with a hydroxy carbamate compound. However, the resulting linear polyurethane polymer may again only include two carbamate functional groups, i.e., one at each end of the backbone or chain. As such, crosslinking density may again be rather low, particularly for higher molecular weight polyurethane polymers used to make aqueous polyurethane-based automotive coating compositions.